Starch synthesis occurs in the chloroplast while soluble carbohydrate (i.e., sucrose) synthesis occurs in the cytosol. These biosynthetic pathways are competing processes because excess triose phosphate can be used for either starch synthesis in the chloroplast or sucrose synthesis in the cytosol. These pathways have many common steps, however, the enzymes that catalyze similar steps are unique to each compartment. These enzymes are isozymes; different forms of the enzymes that catalyze the same reaction. For example, the plastidic and cytosolic forms of phosphoglucomutase both catalyze the conversion of glucose-6-phosphate to glucose 1-phosphate in different subcellular locations.
At maturity, about 40% of soybean seed dry weight is protein and 20% extractable oil. These constitute the economically valuable products of the soybean crop. Of the remaining 40% of seed weight, about 10% is soluble carbohydrate. The soluble carbohydrate portion contributes little to the economic value of soybean seeds and the main component of the soluble carbohydrate fraction, raffnosaccharides, are deleterious both to processing and to the food value of soybean meal in monogastric animals (Coon et al., (1988) Proceedings Soybean Utilization Alternatives, Univ. of Minnesota, pp. 203-211).
It may be possible to modulate the size of the starch and soluble carbohydrate pools in plant cells by altering the catalytic activity of specific enzymes (i.e., phosphoglucomutase) in the starch and soluble carbohydrate biosynthetic pathways (Taiz L., et al. Plant Physiology; The Benjamin/Cummings Publishing Company: New York, 1991). For example, during soybean seed maturation a large portion of the glucose which is converted to soluble carbohydrates (sucrose, raffinose and stachyose) during soybean seed maturation comes from the break down of a starch pool which was produced slowly during the primary growth phase. Elimination of this transient starch pool may be a strategy for diverting carbon away from the soluble carbohydrate components of dry soybean seeds (sucrose, raffinose and stachyose) and into the more economically desirable components such as oil and protein. This strategy may also be applicable to other plants such as corn, rice and wheat.
There is a great deal of interest in identifying the genes that encode proteins involved in starch and soluble carbohydrate biosynthesis in plants. The genes that code for these enzymes may be used to study the interactions among individuals of the pathways and develop methods to alter starch and soluble carbohydrate biosynthesis. Accordingly, the availability of nucleic acid sequences encoding all or a substantial portion of a plastidic or cytosolic phosphoglucomutase enzyme would facilitate studies to better understand starch and soluble carbohydrate biosynthesis in plants and provide genetic tools to enhance or otherwise alter starch and soluble carbohydrate biosynthesis.
The rug3 locus of Pisum sativum encodes the pea plastidic phosphoglucomutase (EP 1001029A1, the entire contents of which are herein incorporated by reference). Pea seeds, of the rug3rug3 genotype, substantially lacking plastidic phosphoglucomutase activity, have higher levels of sucrose at the end of the vining period (EP 1001029A1). High-sucrose soybean lines are known to have better industrial processing and food flavor qualities, in the production of soy protein. Consequently, decreasing or eliminating expression of the plastidic phosphoglucomutase gene in soybeans would be desirable for industrial preparation of soy protein.